Advancements in communication technologies have permitted the creation and implementation of digital communication systems of improved performance. A digital cellular communication system, such as one constructed according to one of the aforementioned Standards, is exemplary of a communication system having improved performance characteristics relative to conventional analog systems.
In such a TDMA (time-division, multiple-access) system, carrier frequencies (hereinafter referred to as "carriers") of a frequency band allocated to such communication systems are divided into time slots. Groups of time slots together form frames. Time slot-carrier combinations define channels upon which bursts of communication signals are transmitted to mobile terminals. Because a plurality of channels are defined upon a single carrier, separate communications can be effectuated with a plurality of mobile stations on a single carrier. Communication signal bursts transmitted by a radio base station are referred to as downlink bursts. And communication signal bursts transmitted by a mobile terminal to a radio base station are referred to as uplink bursts.
A cellular communication system typically makes relatively efficient use of the frequency band allocated thereto. Typically, relatively low-power signals are transmitted. Because relatively low-power signals are transmitted at the same frequencies throughout the geographical areas encompassed by such systems, the same frequencies can be reused. Frequency planning procedures are used to allocate carriers to different cells of the system so that the carriers can be reused in different ones of the cells, but in manners so that co-channel interference do not disrupt communications.
However, as usage of cellular communication systems increases, there is a continuing need to find manners by which to utilize even more efficiently the frequency bands allocated thereto.
Various proposals have been set forth to yet more efficiently utilize the frequency bands allocated for cellular communications. By more efficiently utilizing the frequencies allocated to cellular communications, the communication capacity of the system can be increased at improved signal quality levels.
The aforementioned co-pending patent application, application Ser. No. 08/787,758 discloses, inter alia, the usage of antenna apparatus capable of generating directional antenna beam patterns. Utilization of directional antenna beam patterns to effectuate communications permits separate communications within a single cell to be effectuated concurrently during a single time slot. Formation of directional antenna beam patterns is a form of spatial power control. That is to say, signal power levels of signal bursts broadcast throughout an area encompassed by an antenna beam pattern are significantly attenuated beyond the area encompassed by the antenna beam pattern.
Temporal power control similarly can be used to increase the communication capacity of a cellular communication system. Use of either of such forms of power control permits an increase in the communication capacity of a cellular communication system. Temporal power control refers to the control of the power levels at which communication signal bursts are transmitted. By minimizing the power levels at which communication signal bursts are transmitted, interference caused by the transmission of such communication signal bursts can be minimized.
In some existing cellular communication systems, provision is made for control of the power levels of uplink bursts generated by mobile terminals. But, control of the power levels of downlink bursts is limited. Such limitation is usually caused by requirements that a mobile terminal make use of downlink bursts transmitted on time slots adjacent to the time slot upon which the mobile terminal is to receive a downlink burst.
Adjacent slot information is used generally for purposes of receiver-circuitry, complexity reduction and to facilitate the tracking and synchronizing of a mobile terminal. Such requirements limit the amounts by which control can be exerted over the power levels of downlink bursts transmitted in systems which comply with the IS-136, PDC, and GPRS in GSM Standards. In an existing GSM communication system, downlink power control on a per-slot basis can be applied. In an existing PDC cellular communication system, downlink power control of up to 8 dB between adjacent time slots is permitted. And, in an existing IS-136 system, downlink transmission power levels must be kept at a constant level during a frame when a time slot is active. Requirements related to downlink transmission power levels in the IS-136 system are set forth, inter alia, in Section 3.1.2 of the published version of the IS-136.2-A Standard.
In a mobile terminal operable in an IS-136 system, the mobile terminal uses training sequences included in adjacent time slots for purposes of synchronization and for aiding demodulation. As noted above, presently, the power levels of a downlink transmission must be kept at a constant level if any of the slots defined upon a carrier is active. Hence, no downlink power control on a time slot basis is permitted in an IS-136 system.
In a mobile terminal operable in a PDC communication system, the mobile terminal measures the receive signal strength levels in adjacent slots for purposes of diversity antenna selection. A maximum power reduction of -8 dB relative to the power levels in an active slot is permitted in non-active time slots defined in the PDC communication system.
In a mobile terminal operable to provide GPRS in a GSM communication system, the mobile terminal reads the uplink state flag transmitted in a downlink transmission to be able to determine if the mobile terminal is assigned to use a subsequent uplink time slot for transmission. So, the amount by which power levels of the downlink transmissions can be altered is limited.
If downlink temporal or spatial power control were to be introduced on a per-time slot, time-scale system, performance degradation in systems constructed to be in compliance with any of these three Standards would occur. By exerting better control over the power levels at which the downlink bursts are transmitted, the performance of the communication system can be improved.
A manner by which to provide downlink power control in a TDMA communication system on a time slot-by-time slot basis while also providing a manner to permit a receiving station to extract information from signals transmitted to other receiving stations would therefore be advantageous.
It is in light of this background information related to TDMA communication systems that the significant improvements of the present invention have evolved.